1. Field of the Invention
This invention relates to transversal filters and particularly to an improved transversal filter utilizing charge transfer device techniques and providing subtraction with a substantially high common mode rejection.
2. Description of the Prior Art
CTD transversal filters, either one sided or two sided, have been developed such as the single-sided CCD (charge coupled device) transversal filter of U.S. patent application Ser. No. 960,903 filed Nov. 15, 1978 by Paul R. Prince and assigned to the assignee of the present application. CTDs include bucket brigade devices and CCDs. In the one-sided transversal filter utilizing single phase operation, for example, signal charge packets are all located under the .phi..sub.1 storage electrodes when .phi..sub.1 is low (P-channel device). At t.sub.2 time, a signal .phi..sub.SET clamps positive tap nodes to the voltage .phi..sub.REF+ and all the negative tap nodes to .phi..sub.REF-. The .phi..sub.SET signal is released while .phi..sub.1 is down so that as .phi..sub.1 rises the displacement currents in each group, that is the positive and negative groups of electrodes, are separately summed on their respective nodal capacitances. The resulting positive signals on the two output leads are caused by each charge packet entering under its downstream floating electrode. As the resulting displacement current flows into the nodal capacitance, the signals .DELTA..sub.V+ and .DELTA..sub.V- on the two output leads must then be accurately differenced using a differential or instrumentation amplifier. Utilizing differential amplifiers has been found to present problems of common mode rejection, linearity, noise, bandwidth, power, size, and relative gain. An arrangement to form a transversal filter that eliminates the need for differencing the output signals while eliminating relative gain errors would represent a substantial advance to the art.